Fish meal is an excellent source of protein, and as such has been considered for use as an additive to feed for animals, such as chickens. There is an abundant source of fish for use in making fish meal, as there are often many trash fish caught which are considered unsuitable for human consumption, or only certain parts of the fish are recovered for human use, with the remainder being discarded. This is true for herring, which are caught principally for their roe, the herring in excess of that which can readily be marketed being discarded.
The unusable or unused fish cannot be returned to the sea because dumping of this fish over the side of a vessel may scare away other fish, making the fishing grounds unsuitable for future use. Another problem with disposal of the unused fish is the significant odor, and consequently environmental, problem of dead fish.
Unfortunately, currently available fish meal, even after all of the fish oil has been removed from the adipose tissues by conventional techniques, still contains a significant amount, up to about 5%, of oil. This oil is contained in the cell membranes of the fish meal, mainly as phospholipids. Some of this oil is in the form of polyunsaturated fatty acids, which are extremely vulnerable to oxidation at ambient conditions. The oxidation products of these free polyunsaturated fatty acids contribute to the unpleasant taste and odor of fish meal. Because of this unpleasant taste and odor, such fish meal, which would otherwise be an excellent source of protein as a supplement in food for human consumption or in animal feed, has been substantially eliminated from this market. Furthermore, these oxidized free fatty acids, when present in chicken feed, for example, impart a fishy odor to the chickens themselves and to the eggs that they lay. Additionally, it is known that the consumption of the oxidized products of free fatty acids can cause neurological damage. Great quantities of vitamin E must be added to the fish meal in order to offset the effect of these oxidation products and to decrease to some extent the neurological damage that can be done in order for it to be used as fodder. For these reasons, chicken feed, for example, currently can contain only up to about 5% of fish meal, mixed in with other traditional ingredients of chicken feed such as corn, etc.
Traditionally, fish meal has been prepared by any of various processes such as extracting oil from comminuted fish by cooking the fish to render the oil, extracting the oil with organic solvents, or extracting by cold pressing the fish under pressure or centrifuge, to separate the oil in an organic phase from the proteins in the aqueous phase. These processes do not remove all of the oil from the fish. While they may remove all of the oil readily available in the adipose tissues, they will not remove the oil which is bound to the cell membranes in the form of membrane phospholipids. The cell membranes contain up to 5% of the total fish oil in the form of membrane phospholipids. This amount of oil, which cannot be removed by the above-mentioned methods, is sufficient to make the fish meal substantially unsuitable for use as a protein supplement. The high percentage of polyunsaturated fatty acids, particularly omega-3-polyunsaturated fatty acids such as EPA and DHA, occurring in the membrane phospholipids, while valuable as nutritional supplements, are very vulnerable to oxidation, thus rendering the fish meal unsuitable for use as a source of protein and difficult to dispose of.
Many methods for removing oil from fish or fish meal have been proposed in the prior art. However, none of these methods specifically address the problem of oils present in membrane phospholipids, and many of these processes subject the fish to extremes of temperature that may cause formation of undesirable by-products. Among the problems encountered in subjecting fish to elevated temperatures is that the elevated temperatures may cause rancidity, and thus undesirable odors, of the fatty acids present therein. Additionally, particularly in view of the presence of decarboxylase enzymes in the cells themselves, amino acids present in the fish may be decarboxylated to amines, many of which have disgusting odors.
Vogel et al., in U.S. Pat. No. 2,875,061, disclose a process for preparing protein products from fish materials wherein raw fish are first subjected to fat extraction by an organic solvent and then heated in a solution of alkali hydroxide in ethanol to remove fishy taste and smell from the fish material. The process of this patent does not involve conditions which will inherently hydrolyze phospholipids. Furthermore, there is no provision to recover any valuable free polyunsaturated fatty acids.
Roels, U.S. Pat. No. 3,836,686, teaches a process for recovering a protein hydrolysate having an oil concentration of less than 0.15% from whole fish or fish products by digesting a homogenized slurry of the fish at elevated temperature in an acid medium at a pH of about 1 and separating and purifying the aqueous phase of the digestion step. This high temperature acid treatment provides undesirable side effects in that the amino acids become decarboxylated to amines which have undesirable smell and a moderate degree of toxicity.
Danish patent No. 141,922 extracts oil and protein material from fish or fish entrails or livers by mixing with sulfuric acid to a ph of 1.2 to 2.2 and maintaining at 40.degree. C. for 1 to 4 days and then neutralizing and centrifuging to obtain three fractions, fats, a protein hydrolysate and a sludge. The conditions of this process are not sufficient to hydrolyze cell membrane phospholipids.
European patent No. 169,166 discloses a method of hydrolysis of animal or vegetable proteins by treatment with gaseous hydrochloric acid. The intent of this patent is ony to hydrolyze the proteins and there is no disclosure of hydrolyzing phospholipids in order to remove all fatty acids.
Suzuki, in U.S. Pat. No. 4,060,642, discloses a method for making a concentrated food product from the meat of marine animals by adding to the marine animal meat an edible salt to adjust the pH to about 6.5-7.7, and removing fats and water by use of a hydrophilic organic solvent.
Faith et al., in U.S. Pat. No. 3,607,285, disclose a process for solubilizing and extracting protein from fish, fish products, or fish parts by treating the fish with alkaline bacterial protease. This enzyme works on the protein and not on membrane phospholipids.
Conn, in U.S. Pat. No. 1,903,503, discloses a process for extracting oil and moisture from fish press cake by treating the fish cake with petroleum ether or other volatile solvent and removing excess solvent with a vacuum pump.
Sahlstrom, in U.S. Pat. No. 353,822, discloses a process for making extracts from fish wherein the fish is treated with hypermanganic alkali and ammonia solution, and then centrifuged, to remove fat from the fish. The oil is then separated for special treatment, which treatment is not disclosed.
Herwig, in U.S. Pat. No. 547,548, discloses a process for preparing fish meal by heating comminuted fish under dry heat to cause part of the water in the tissue to be vaporized, thereby bursting the cell walls of the fish flesh to lay open juices. These juices are evaporated with heat, and the oil is removed by means of organic solvents. No means to remove oil from the phospholipids bound to the cell membranes is disclosed.
None of the prior art workers have recognized that a specifically non-oily fish meal cannot be obtained without liberating the oily material from the membranes of the fish flesh and that this oil must be liberated in a manner which does not cause degradation of the amino acids.